Audio production equipment advantageously includes meters or readouts whereby an operator can determine various conditions that are pertinent to processing of the signal content, quality and other concerns. The moving display of a meter can provide information that is useful for various purposes and not only for the assurance that there is a signal present. A meter may provide a visual warning that the signal amplitude may be too high and may potentially cause clipping or distortion by overdriving audio amplifiers. The signal level may be low, potentially introducing hiss. Apart from signal amplitude, meters may be provided as indicators for other parameters, such as frequency spectrum, carrier modulation in a transmitter, etc. In this context, a “meter” might entail any of various changeable indicators such as movable pointers, a variable line of lamps or LEDs, changeable colors, and other indicating techniques.
In connection with amplitude meters, one conventional amplitude meter is the VU meter (an acronym for “volume units”) and typically responds to an average level (such as the RMS average) over some sampling period. Another conventional type of meter is the PPM or program peak meter. The VU meter responds more slowly to changes in the signal amplitude than the PPM or peak meter, the response rate of the meter being sometimes termed the meter “ballistics.”
VU and peak meters each produce valid information in their own way. For example, a VU meter gives a good measure of the loudness of a signal as subjectively perceived by humans. A peak meter gives a better indication as to whether amplifiers are being overdriven and so as to deteriorate the signal quality. A simple peak meter may have the disadvantage that in order to respond quickly to peaks, the meter cannot persist very long in displaying a signal level.
Meters for monitoring levels such as sound levels may have threshold detection aspects. In a meter with a graduated scale, the scale may have a “red line” area where the signal amplitude is considered too high. In a digital display having a line of LEDs or other individually controlled indicators to represent a variable line length, the indicators may have color warning attributes such as green for OK and red for overdriven.
A single meter may accommodate two types of meter ballistics in one display. Attempts to represent two or more variables in a display may be useful. The variables advantageously should be related to one another, and even though related, should be represented by distinctly different aspects of the display in some logical way. For example, peak and VU levels both represent signal amplitude. The peak indication can be represented by a distinct color and can be represented by a providing an indicator with a longer time of persistence of a peak indicator LED compared to a VU indicating one, which is curious because the shorter term variable (peak) is given a longer persistence than the longer term variable (VU).
A meter display or other presentation of data can be made unduly complicated. In a case where there is an effort to represent plural variables using different changeable display attributes, changes occur at the same time, possibly obscuring information in the complexity. Audio meter complexity is multiplied when attempting to provide indicators that represent multiple channels of audio. In particular, indicators lose much of their effectiveness in conveying information when three or more channels are involved.
In a stereo system, with two separate audio channels, the content of the channels might be different, but frequently stereo content has a directional or spatial aspect. For example the volume on two stereo channels might be varied to provide a realistic audio presentation of sound sources that should seem louder on one of the left and right side than the other, or sources that sound as though they are moving from left to right or vice versa. As a result, issues of phase relationship, relative volume and channel separation arise and may be pertinent in test equipment for use in producing, transmitting, or otherwise processing program content.
Test equipment that can usefully display a representation of differences in volume and phase at spaced audio speakers becomes involved when one has multiple channels, such as three or more channels, because the issues are no longer simply a matter of dual sources and a balance and/or phase relationship between the two sources. Digital video systems conventionally can have eight distinct channels. In comparison with a two channel stereo system in which one might be concerned with volume and left/right balance and phase relationship, in a multi-channel system one might want to compare any subset of one or more of the channels to any other subset of one or more channels.
With respect to the phase relationship between two signals such as a left and right audio channel, it is known to use a Lissajous display as an indicator. According to this technique, two periodic signals that are to be compared are applied to the horizontal and vertical inputs of a deflection apparatus such as an oscilloscope (or an analogous device). If the signals are equal in phase, a diagonal line is displayed. A static phase difference produces an oval or circle. Different frequencies produce more complex patterns and the patterns persist or change with changes in phase. This sort of a display becomes extremely cluttered by complex real world signals, particularly if the signals have a high degree of channel separation. Thus, the Lissajous display is often primarily useful as a visual indicator of the extent of channel separation.
In connect with audio systems having multiple channels (three or more), the complexity of highly variable visual displays presents a challenge. A meter or test apparatus with a visual display that is inherently complex becomes many times more complex with the presence of multiple channels because various comparisons are possible.
In a surround sound arrangement, an audio program conventionally has at least five channels, namely Left, Right, Center, Left Rear and Right Rear. Any pair of related channels (Left versus Right, Front versus Rear, etc.) presents the balance, separation and phasing issues comparable to two stereo channels. What is needed is a way to show the relationships among all the channels in a logical and intuitive form whereby a viewer can easily understand and work with the audio, such as monitoring attributes and adjusting hardware devices that are processing it, as necessary. It is possible envision a left/right balance display and a front/rear fade display that is complementary with balance and fade controls, but what would be more helpful and versatile is a test apparatus that is not limited to left/right and/or front/rear, and instead is capable of displays that present pertinent information as to any channel relationship and for a larger number of channels, such as eight channels. Such a versatile display would be useful apart from present rigid assignments between channels and speaker positions.
However many variations might be needed to enable a display to usefully compare all combinations of single channels or pairs if there are multiple channels available. These variations, multiplied by the complexity of selective pairings of channels and the possible need simultaneously to display relative amplitude or balance and relative phase information, could make a test apparatus display substantially too complex and difficult to understand, especially for persons who are not very familiar with the display. What is needed in a display is a presentation of information that is highly intuitive and provides a visual indication that clearly and logically provides the user with information needed about various audio parameters.